This invention relates to a platen drive device for use in a printer, an electronic typewriter or the like.
A platen drive device has conventionally been used to drive a platen of a printer, an electronic typewriter or the like.
Such a conventional platen drive device will now be described. FIG. 4 is a side-elevational view of the conventional platen drive device. A drive gear 22 is mounted on a shaft of a motor 21, and a gear 24 having a worm 23 integrally formed therewith is in mesh with the drive gear 22. As best shown in FIG. 5, the worm 23 has teeth formed on a part of its outer peripheral surface, the teeth (hereinafter referred to as "spiral teeth") being disposed on a spiral line. A teeth-free portion, i.e. a portion at which the spiral teeth are cut off or removed is provided on the outer peripheral surface of the worm 23. A reflection plate 24a is mounted on one side or face of the gear 24. A worm gear 25 is engageable with the spiral teeth of the worm 23, and also is in mesh with teeth of a ratchet 27 mounted coaxially with a platen 26. An arm 29 is urged against the teeth of the ratchet 27. A photosensor 30 is fixedly mounted in such a position as to be opposed to the reflection plate 24a when the worm 23 is disengaged from the worm gear 25. A motor braking judgment portion 31 feeds a braking signal to a motor driver 32 (which drives the motor 21) when a detection signal is outputted from the photosensor 30.
The operation of the above platen drive device will now be described.
First, when the motor 21 is in its stopped condition, the worm 23 and the worm gear 25 are stationary, and are not engaged or meshed with each other. In this condition, when the motor 21 begins to rotate, the worm 23 is soon brought into meshing engagement with the worm gear 25, so that the worm gear 25 begins to thereby rotate to drive the platen 26. Then, the worm 23 is further rotated, and when the worm 23 is disengaged from the worm gear 25, the worm gear 25 is stopped. At this time, the platen 26 is retained in this position by the engagement of the arm 29 with the ratchet 27. Also, at this time, the photosensor 30 outputs a detection signal, and the motor braking judgment portion 31 is responsive to this detection signal to feed a stop signal to the motor driver 32, so that the motor driver 32 stops the motor 21. At this time, the stop position of the motor is irregular. However, if the range of this position irregularity is arranged to be within the range of the teeth-free portion of the worm 23, the platen can be angularly moved a certain amount regardless of the stop position of the motor 21. And besides, the only means for retaining the platen 26 in a braked condition when the motor 21 is stopped is the ratchet mechanism constituted by the ratchet 27 and the arm 29, and therefore, the platen 26 can be manually rotated.
However, with the above conventional construction, the plurality of teeth of the worm are intermittently engaged or meshed with the plurality of teeth of the worm gear, and therefore if the relation between the engaging position of the ratchet mechanism and the engaging position of the worm gear is not constant, the position of the worm gear is changed after the worm is disengaged from the worm gear. In such a case, the next engagement of the worm with the worm gear can not be done properly. For this reason, the design of the ratchet mechanism is difficult, and the manufacturing precision of the ratchet mechanism as well as the manufacturing precision of the worm and the worm gear must be enhanced. As a result, the cost has been increased.